1. Technical Field
The present invention relates to a wire bonding method and apparatus for connecting a wire to a first conductor and a bump on a second conductor after such a bump has been formed on the second conductor.
2. Description of the Related Art
In wire bonding, since a ball formed on the tip end of the wire is bonded to the first conductor as a primary bonding, the joining properties are good. However, to the second conductor, since the wire without a ball is bonded as a secondary bonding, the joining properties are poor compared to the joining properties of the primary bonding. Accordingly, a unique technique is used, in which a bump is formed on the second conductor, and then the wire is connected between the first conductor and the bump on the second conductor.
Japanese Patent Application Laid-Open (Kokai) Nos. H10-112471 and 2002-280410, for instance, respectively disclose such a wire bonding method as described above, and it is described in FIG. 3.
As shown in FIG. 3(a), a die 2 on which a bump(s) 2a is formed is mounted on a circuit board 1, which is a lead frame or a board such as a ceramic board or printed board, etc. Wiring 3 is also formed on the circuit board 1. A bump(s) 10 is formed on the wiring 3 beforehand by a wire bonding apparatus. The upper surface of the bump 10 has an inclined surface 12 that is formed so as to be inclined in the opposite direction from the die 2 (first conductor). Accordingly, after a ball is formed on the tip end of the wire 4 by means of an electric torch (not shown), the capillary 5 is lowered and primary bonding is performed on the pad 2a of the die 2 as shown in FIG. 3(a); next, as shown in FIG. 3(b), looping of the wire 4 is performed, the wire 4 is positioned on the upper part of the inclined surface 12 of the bump 10, and secondary bonding of the wire 4 to the inclined surface 12 is performed. Subsequently, the wire 4 is cut.
In the example of FIG. 4, a bump 10 is formed beforehand on the pad 2a, and the inclined surface 12 of the bump 10 is formed on the opposite side from the wiring 3. Accordingly, primary bonding is performed on the wiring 3, secondary bonding is performed on the inclined surface 12 of the bump 10, and the wire 4 is then cut.
The wire bonding apparatus that performs the shaping of the bumps 10 and the connection of the wire 4 has a structure shown in FIG. 5.
A bonding arm 20 which has a capillary 5 on its one end is fastened to one end of a supporting frame 21. The supporting frame 21 is attached to a moving table 23 via a supporting shaft (not shown in the drawings) or a plate spring 22 assembled in a cruciform configuration so that this supporting frame 21 is free to swing upward and downward, and the moving table 23 is mounted on an XY table 24. The coil 26 of a linear motor 25 is fastened to the other end of the supporting frame 21, and the magnet 27 of this linear motor 25 is fastened to the moving table 23. A linear scale 28 is fastened to the rear end of the supporting frame 21, and a position sensor 29 is fastened to the moving table 23 so as to face this linear scale 28. The wire bonding apparatus further has a heater block 31 that heats the device 30. The heater block 31 is raised and lowered by a raising-and-lowering mechanism 32.
Japanese Patent Application Laid-Open (Kokai) Nos. S58-184734, H6-29343 and S61-163648 (Publication (Kokoku) No. H6-80697), for instance, disclose the wire bonding apparatus of the above-described type.
In structure described above, the supporting frame 21 and bonding arm 20 are caused to swing about the supporting shaft or plate spring 22 by the linear motor 25, so that the capillary 5 is raised and lowered. The moving table 23, supporting frame 21, bonding arm 20 and capillary 5 are moved in the X and Y directions by the XY table 24. The bumps 10 shown in FIGS. 3(a) and 3(b) and 4 are formed on the device 30 by the formation of a ball on the tip end of the wire 4 by an electric torch (not shown in the drawings), the raising and lowering movement of the capillary 5, and the opening-and-closing movement of wire cutting clampers (not shown in the drawings) during the cutting of the wire, etc. The wire 4 is connected to the device 30 as shown in FIGS. 3(a) and 3(b) and 4 by the formation of a ball on the tip end of the wire 4 by the electric torch (not shown in the drawings), a combination of upward and downward movement and movement in the X and Y direction of the capillary 5, and the opening-and-closing movement of the wire cutting clampers (not shown in the drawings) during the cutting of the wire, etc.
Next, a control of the linear motor 25 and actions of the respective blocks will be described.
The input-output of various types of information required for operation to and from the computer 41 is performed by external input-output means 40. This can be accomplished by manual operation or by operation based on-line communications with an external device. The computer 41 has a control circuit 42, a calculating circuit 43 and a height position counter 44; and the control circuit 42 controls the external input-output means 40, the calculating circuit 43 and a position control circuit 50.
When a height position command for the capillary 5 is inputted into the position control circuit 50 from the control circuit 42, the position control circuit 50 transmits the amount of movement of the capillary 5 to a motor driver 51 as a driving signal 50a. In accordance with this driving signal 50a, the motor driver 51 generates electric power that is used to move the capillary 5 to the designated position. Since such electric power is a product of voltage and current, the actual control of the linear motor 25 can be accomplished by controlling either the voltage or current, or both. Here, the system will be described in terms of the driving current 51a that flows to the linear motor 25. When the driving current 51a produced by the motor driver 51 is applied to the coil 26 of the linear motor 25, a driving force is generated. The supporting frame 21, bonding arm 20 and capillary 5 are caused to swing about the plate spring 22 by this driving force.
The height position counter 44 counts signals from an encoder 52 that converts the signal from the position sensor 29 into a signal format that is suitable for inputting into the computer 41, and also the height position counter 44 generates the actual height position of the linear scale 28. The computer 41 is inputted beforehand with the ratio of the amount of movement of the capillary 5 in the vertical direction to the amount of movement of the linear scale 28 in the vertical direction and with a quantization coefficient (one unit equals several microns). Accordingly, the computer 41 determines the actual height position of the capillary 5 by performing a mathematical operation (by means of the calculating circuit 43) on the value indicated by the height position counter 44 on the basis of the above-described ratio and quantization coefficient.
In cases where no bump 10 is formed on the second conductor, the joining strength between the wire 4 and second conductor 2 in the secondary bonding is poor. Accordingly, it is necessary to examine whether or not a bump 10 is normally formed on the second conductor.
Japanese Patent Application Laid-Open (Kokai) No. 2000-306940, for example, discloses inspection for bump if it is formed or not formed (or adhered) at the time of bump formation.
In Japanese Patent Application Laid-Open (Kokai) No. 2000-306940, with a utilization of the time constant of the total impedance of the device, wire and internal parts of the circuit during monitoring of the state of electrical continuity with a voltage applied, a voltage is applied to the wire during the period which is from the connection of the ball to the conductor to the cutting of the wire from the attachment root of the ball, and the presence or absence of ball non-adhesion is ascertained by detecting variations in the voltage during the above-described period according to the presence or absence of ball non-adhesion, and specifically by detecting the delay time in the rise of the voltage.
Since bump not-adhered can be detected by the method of Japanese Patent Application Laid-Open (Kokai) No. 2000-306940 at the time of bump formation, devices on which a bump is not adhered should not be sent into subsequent processes. However, in this prior art, no great difference is generated in the voltage rise time between the state in which the wire is normally cut (tail cutting) following the raising of the capillary and the state in which no bump is adhered. Accordingly, there is a danger that a “bump not-adhered” situation cannot be judged in a normal fashion. Currently, therefore, devices on which bumps are formed are subjected to an external appearance inspection, which is conducted for all of the devices produced, by an image processing device.